At first, Huck isn't quite sure what kind of bad luck will happen, but since he's a superstitious type, apparently spilling salt brings bad luck. So in chapter 4 after he tipped it over, he went about "feeling all worried and shaky, and wondering where [the bad luck] was going to fall on" him. It is right after this that he discovers his pap's shoeprint in the snow. Being the rather smart kid that he is, he is a bit worried that Pap is there, and will cause trouble, so he goes straight over to Judge Thatcher's to sell all of his money to him. He knows that if he has that money, his Pap will come after it, and get it. So Thatcher buys the money from him for $1, and then Huck goes to get a fortune told from Jim's oxen hairball. He is worried about Pap and "what he was going to do, and was he going to stay"? The "hairball" doesn't mention anything about his pap, but sure enough, in chapter 5, Pap shows up. This is the bad luck that resulted from the salt-spilling, and from then on out, it's just trouble and heartache for Huck, until he manages to escape the clutches of his drunken pap.